Higher operating temperatures for gas turbine engines are continuously sought in order to increase their efficiency. However, as operating temperatures increase, the high temperature durability of the components of the engine must correspondingly increase. While significant advances have been achieved with iron, nickel and cobalt-base superalloys, the high-temperature capabilities of these alloys alone are often inadequate for components located in certain sections of a gas turbine engine, such as the turbine, combustor and augmentor. A common solution is to thermally insulate such components in order to minimize their service temperatures. For this purpose, thermal barrier coatings (TBC) formed on the exposed surfaces of high temperature components have found wide use.
To be effective, thermal barrier coatings must have low thermal conductivity and adhere well to the component surface. Various ceramic materials have been employed as the TBC, particularly zirconia (ZrO2) stabilized by yttria (Y2O3), magnesia (MgO) or other oxides. These particular materials are widely employed in the art because they can be readily deposited by plasma spray and vapor deposition techniques. An example of the latter is electron beam physical vapor deposition (EBPVD), which produces a thermal barrier coating having a columnar grain structure that is able to expand with its underlying substrate without causing damaging stresses that lead to spallation, and therefore exhibits enhanced strain tolerance. Adhesion of the TBC to the component is often further enhanced by the presence of a metallic bond coat, such as a diffusion aluminide or an oxidation-resistant alloy such as MCrAlY, where M is iron, cobalt and/or nickel.
Processes for producing TBC by EBPVD generally entail preheating a component to an acceptable coating temperature, and then inserting the component into a heated coating chamber maintained at a pressure of about 0.005 mbar. Higher pressures are avoided because control of the electron beam is more difficult at pressures above about 0.005 mbar, with erratic operation being reported at coating chamber pressures above 0.010 mbar. It has also been believed that the life of the electron beam gun filament would be reduced or the gun contaminated if operated at pressures above 0.005 mbar. The component is supported in proximity to an ingot of the ceramic coating material (e.g., YSZ), and an electron beam is projected onto the ingot so as to melt the surface of the ingot and produce a vapor of the coating material that deposits onto the component.
The temperature range within which EBPVD processes can be performed depends in part on the compositions of the component and the coating material. A minimum process temperature is generally established to ensure the coating material will suitably evaporate and deposit on the component, while a maximum process temperature is generally established to avoid microstructural damage to the article. Throughout the deposition process, the temperature within the coating chamber continues to rise as a result of the electron beam and the presence of a molten pool of the coating material. As a result, EBPVD coating processes are often initiated near the targeted minimum process temperature and then terminated when the coating chamber nears the maximum process temperature, at which time the coating chamber is cooled and cleaned to remove coating material that has deposited on the interior walls of the coating chamber. Advanced EBPVD apparatuses permit removal of coated components from the coating chamber and replacement with preheated uncoated components without shutting down the apparatus, so that a continuous operation is achieved. The continuous operation of the apparatus during this time can be termed a “campaign,” with greater numbers of components successfully coated during the campaign corresponding to greater processing and economic efficiencies.
In view of the above, there is considerable motivation to increase the number of components that can be coated within a single campaign, reduce the amount of time required to introduce and remove components from the coating chamber, and reduce the amount of time required to perform maintenance on the apparatus between campaigns. However, limitations of the prior art are often the result of the relatively narrow range of acceptable coating temperatures, the complexity of moving extremely hot components into and out of the coating chamber, and the difficulties confronted when maintaining an advanced EBPVD apparatus. Accordingly, improved EBPVD apparatuses and processes are continuously being sought for depositing coatings, and particularly ceramic coatings such as TBCs.